# Copyright 2019 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================
"""Functions and classes related to optimization (weight updates)."""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import re

import tensorflow as tf


class WarmUp(tf.keras.optimizers.schedules.LearningRateSchedule):
    """Applys a warmup schedule on a given learning rate decay schedule."""

    def __init__(
            self,
            initial_learning_rate,
            decay_schedule_fn,
            warmup_steps,
            power=1.0,
            name=None):
        super(WarmUp, self).__init__()
        self.initial_learning_rate = initial_learning_rate
        self.warmup_steps = warmup_steps
        self.power = power
        self.decay_schedule_fn = decay_schedule_fn
        self.name = name

    def __call__(self, step):
        with tf.name_scope(self.name or 'WarmUp') as name:
            # Implements polynomial warmup. i.e., if global_step < warmup_steps, the
            # learning rate will be `global_step/num_warmup_steps * init_lr`.
            global_step_float = tf.cast(step, tf.float32)
            warmup_steps_float = tf.cast(self.warmup_steps, tf.float32)
            warmup_percent_done = global_step_float / warmup_steps_float
            warmup_learning_rate = (
                    self.initial_learning_rate *
                    tf.math.pow(warmup_percent_done, self.power))
            return tf.cond(global_step_float < warmup_steps_float,
                           lambda: warmup_learning_rate,
                           lambda: self.decay_schedule_fn(step),
                           name=name)

    def get_config(self):
        return {
            'initial_learning_rate': self.initial_learning_rate,
            'decay_schedule_fn': self.decay_schedule_fn,
            'warmup_steps': self.warmup_steps,
            'power': self.power,
            'name': self.name
        }


def create_optimizer(init_lr, num_train_steps, num_warmup_steps):
    """Creates an optimizer with learning rate schedule.

    Args:
      init_lr: 
      num_train_steps: 
      num_warmup_steps: 

    Returns:

    """
    # Implements linear decay of the learning rate.
    learning_rate_fn = tf.keras.optimizers.schedules.PolynomialDecay(
        initial_learning_rate=init_lr,
        decay_steps=num_train_steps,
        end_learning_rate=0.0)
    if num_warmup_steps:
        learning_rate_fn = WarmUp(initial_learning_rate=init_lr,
                                  decay_schedule_fn=learning_rate_fn,
                                  warmup_steps=num_warmup_steps)
    optimizer = AdamWeightDecay(
        learning_rate=learning_rate_fn,
        weight_decay_rate=0.01,
        beta_1=0.9,
        beta_2=0.999,
        epsilon=1e-6,
        exclude_from_weight_decay=['layer_norm', 'bias'])
    return optimizer


class AdamWeightDecay(tf.keras.optimizers.Adam):
    """Adam enables L2 weight decay and clip_by_global_norm on gradients.
    
      Just adding the square of the weights to the loss function is *not* the
      correct way of using L2 regularization/weight decay with Adam, since that will
      interact with the m and v parameters in strange ways.
    
      Instead we want ot decay the weights in a manner that doesn't interact with
      the m/v parameters. This is equivalent to adding the square of the weights to
      the loss with plain (non-momentum) SGD.

    Args:

    Returns:

    """

    def __init__(self,
                 learning_rate=0.001,
                 beta_1=0.9,
                 beta_2=0.999,
                 epsilon=1e-7,
                 amsgrad=False,
                 weight_decay_rate=0.0,
                 include_in_weight_decay=None,
                 exclude_from_weight_decay=None,
                 name='AdamWeightDecay',
                 **kwargs):
        super(AdamWeightDecay, self).__init__(
            learning_rate, beta_1, beta_2, epsilon, amsgrad, name, **kwargs)
        self.weight_decay_rate = weight_decay_rate
        self._include_in_weight_decay = include_in_weight_decay
        self._exclude_from_weight_decay = exclude_from_weight_decay

    @classmethod
    def from_config(cls, config):
        """Creates an optimizer from its config with WarmUp custom object.

        Args:
          config:

        Returns:

        """
        custom_objects = {'WarmUp': WarmUp}
        return super(AdamWeightDecay, cls).from_config(
            config, custom_objects=custom_objects)

    def _prepare_local(self, var_device, var_dtype, apply_state):
        super(AdamWeightDecay, self)._prepare_local(var_device, var_dtype,
                                                    apply_state)
        apply_state['weight_decay_rate'] = tf.constant(
            self.weight_decay_rate, name='adam_weight_decay_rate')

    def _decay_weights_op(self, var, learning_rate, apply_state):
        do_decay = self._do_use_weight_decay(var.name)
        if do_decay:
            return var.assign_sub(
                learning_rate * var *
                apply_state['weight_decay_rate'],
                use_locking=self._use_locking)
        return tf.no_op()

    def apply_gradients(self, grads_and_vars, name=None):
        grads, tvars = list(zip(*grads_and_vars))
        (grads, _) = tf.clip_by_global_norm(grads, clip_norm=1.0)
        return super(AdamWeightDecay, self).apply_gradients(zip(grads, tvars))

    def _get_lr(self, var_device, var_dtype, apply_state):
        """Retrieves the learning rate with the given state.

        Args:
          var_device:
          var_dtype:
          apply_state:

        Returns:

        """
        if apply_state is None:
            return self._decayed_lr_t[var_dtype], {}

        apply_state = apply_state or {}
        coefficients = apply_state.get((var_device, var_dtype))
        if coefficients is None:
            coefficients = self._fallback_apply_state(var_device, var_dtype)
            apply_state[(var_device, var_dtype)] = coefficients

        return coefficients['lr_t'], dict(apply_state=apply_state)

    def _resource_apply_dense(self, grad, var, apply_state=None):
        lr_t, kwargs = self._get_lr(var.device, var.dtype.base_dtype, apply_state)
        decay = self._decay_weights_op(var, lr_t, apply_state)
        with tf.control_dependencies([decay]):
            return super(AdamWeightDecay, self)._resource_apply_dense(
                grad, var, **kwargs)

    def _resource_apply_sparse(self, grad, var, indices, apply_state=None):
        lr_t, kwargs = self._get_lr(var.device, var.dtype.base_dtype, apply_state)
        decay = self._decay_weights_op(var, lr_t, apply_state)
        with tf.control_dependencies([decay]):
            return super(AdamWeightDecay, self)._resource_apply_sparse(
                grad, var, indices, **kwargs)

    def get_config(self):
        config = super(AdamWeightDecay, self).get_config()
        config.update({
            'weight_decay_rate': self.weight_decay_rate,
        })
        return config

    def _do_use_weight_decay(self, param_name):
        """Whether to use L2 weight decay for `param_name`.

        Args:
          param_name:

        Returns:

        """
        if self.weight_decay_rate == 0:
            return False

        if self._include_in_weight_decay:
            for r in self._include_in_weight_decay:
                if re.search(r, param_name) is not None:
                    return True

        if self._exclude_from_weight_decay:
            for r in self._exclude_from_weight_decay:
                if re.search(r, param_name) is not None:
                    return False
        return True

    def apply_gradients(self, grads_and_vars, name=None, **kwargs):
        grads, tvars = list(zip(*grads_and_vars))
        return super(AdamWeightDecay, self).apply_gradients(zip(grads, tvars), name=name, **kwargs)
